Lithographic processes involve establishing image (printing) and non-image (non-printing) areas on a substrate, substantially on a common plane. When such processes are used in the printing industries, non-image areas are generally hydrophilic, and image areas are generally oleophilic. Consequently, oil based inks are repelled from the non-image areas after water has been applied to the substrate.
Image and non-image areas can be created by processes which include a step of exposing a layer of image material on the surface of the substrate to radiation. The exposure to radiation creates solubility differences in the image material corresponding to image and non-image areas. Following development, the soluble areas are removed, leaving a pattern on the substrate corresponding to the image.
Preparation of the substrate for receiving a layer of the image material must ensure that the material bonds to the substrate, at least prior to image formation. However, it must allow release of the soluble image material after development.
Suitable image materials for use in lithographic processes can include those based on diazonium/diazide materials, polymers which undergo depolymerization or addition photo-polymerization, and silver halide gelatin assemblies. Examples of suitable materials are disclosed in GB-1592281, GB-A-2031442, GB-A-2069164, GB-A-2080964, GB-A2109573 and EP-A-377589.
Substrates used in the printing industry commonly comprise an aluminum base layer, which has a layer of aluminum oxide on its surface, intermediate the base material and a subsequently applied image layer, resulting from a controlled oxidation reaction conducted electrochemically. Prior to the oxidation reaction, the surface of the aluminum base layer is subjected to a cleaning treatment, for example involving washing with alkali. The base layer is then subjected to a texture control treatment, for example involving an etching process, which increases the surface area of the substrate, which in turn controls the strength of the bond between the substrate and the image material and increases the ability of the substrate to hold water. It can be appropriate in some circumstances to modify the characteristics of the surface coating of aluminum oxide in order to ensure that the strength of the bond between the surface and the subsequently applied image layer is appropriate, both in the regions which are to remain bonded to the oxide coating and in the regions which are to be removed. This treatment can involve treatment with water, a solution of a phosphate or silicate salt, or a polycarboxylic acid.
A problem with this known process for preparing substrates for use in lithographic processes with an aluminum base layer, is that it uses a significant amount of energy in the course of the steps by which the base layer is etched and then oxidized. There can also be significant consumption of materials for cleaning and etching the surface of the substrate prior to oxidation, which gives rise to expense both in terms of the materials themselves and of subsequent disposal of the used materials.
GB-1238701, published in 1971, discloses a process for preparing a lithographic printing plate in which a surface of a foil such as of aluminum is subjected to a uniform treatment with a plasma arc jet. Finely dispersed silica introduced into the jet causes a layer of silicate to be formed on the surface of the foil. The process disclosed in the document does not appear to have been used commercially to produce plates.
Substrates used in lithography can also be formed from materials other than aluminum, such as for example, another metal such as steel, a polymeric material such as a polyester, or a paper based material. Processes used to prepare such substrates for coating with light sensitive material vary widely from that used to prepare an aluminum substrate.